Barrett's esophagus (BE) is a condition where normal esophageal squamous mucosa changes to metaplastic columnar mucosa in response to chronic acid reflux. BE can progress to esophageal adenocarcinoma (EAC), a prevalent (~12,000 cases per year) and deadly cancer (~15% 5-year mortality rate). Current BE management strategies of anti-reflux therapy, endoscopic screening, and regular endoscopic biopsy surveillance and ablation therapy, has been largely ineffective for decreasing the mortality of EAC. One important reason for the failure of the standard of care is our lack of understanding of the microscopic natural history/evolution of BE which has led to controversies about its definition, uncertainty regarding its progression and clinical significance, and an inability to evaluate the effectiveness of pharmacologic and ablative therapies. These questions have arisen in part because the only method used to study BE is endoscopic biopsy, which is invasive, costly, and suffers from severe biopsy sampling limitations. Our laboratory has developed a new imaging concept termed tethered capsule endomicroscopy (TCE) that overcomes these limitations of endoscopic biopsy. TCE involves swallowing a tethered capsule that acquires three-dimensional microscopic images of the entire esophageal wall as it traverses the luminal organ via peristalsis or is pulled up towards the mouth using the tether. As opposed to endoscopy, TCE mitigates sampling error by evaluating the microscopic structure of the entire esophagus. The TCE procedure can be conducted in unsedated patients, making it simpler, faster, better tolerated, and less expensive than endoscopy. The first TCE prototype that we have demonstrated uses optical coherence tomography (OCT) to obtain cross-sectional images of the microscopic architecture of the esophagus at a resolution of 10 ?m. In this proposal, we will advance TCE technology further by combining OCT with a high-speed reflectance confocal microscopy technique called spectrally encoded confocal microscopy (SECM) so that obtain both architectural and cellular morphologic information can be obtained from the same device. We will additionally incorporate a position sensor that will allow microscopic datasets acquired from the same patients at different time points to be registered to one another. Once this device is developed, we will utilize TCE in a multicenter (5-site) clinical trial to study the natural history of BE in 500 patients. Imaging will be conducted on a yearly basis in all patients for a duration of 3 years (1500 patient-years) and the TCE images will be evaluated to determine how BE changes over time. The results of this study will significantly contribute to the knowledge of BE, allowing us to answer longstanding questions about its microscopic definition, its progression and clinical significance, and the effectiveness of ablativ BE therapies. The knowledge gained from this study will be used to improve BE management strategies based on strong scientific evidence about the natural history of this disease.